Separation of black liquor to produce lignins substantially free of sodium

ABSTRACT

The method described herein will produce depolymerized lignins substantially free from sodium compounds. Black liquor supplied from a pulping process is utilized for providing a mixture of depolymerized lignins dispersed and dissolved in a solution containing sodium hydroxide. The mixture is then separated to form depolymerized lignins and a solution containing sodium hydroxide. Following separation, the depolymerized lignins are then extracted with water to produce an extractate containing sodium compounds. The extracted depolymerized lignins, containing sodium compounds, are then reacted with an acid to form sodium salts to form a slurry. The slurry containing sodium salts is, by means of a salt splitting method, separated from the extracted depolymerized lignins to produce substantially sodium free depolymerized lignins. The separated solution containing sodium hydroxide is combined with the extractate, and the combination, subsequent to removal of water, is suitable for recycle to depolymerize, disperse and dissolve additional lignins. As a result, depolymerized lignins substantially free of sodium compounds are produced, in addition to a solution to produce black liquor.

BACKGROUND OF THE INVENTION

[0001] Pulping processes are employed to separate lignins from wood to produce a pulp. A solution containing sodium hydroxide is employed to dissolve lignins from the wood to yield a black liquor containing sodium hydroxide and depolymerized lignins. A recovery boiler is used to recover the sodium hydroxide and other compounds for reuse and release of the energy contained in the lignins to provide steam.

[0002] A state of the art Kraft pulping digestion process employs a solution containing sodium hydroxide and sodium sulfide to create a black liquor. A similar process, termed soda pulping, employs a solution containing sodium hydroxide, devoid of sodium sulfide, to create a black liquor. After concentration of the black liquor, by means of a multiple-effect evaporator, the concentrated black liquor is sent to a recovery boiler to produce steam and a molten smelt containing sodium carbonate. The smelt, of melted sodium compounds, flows to a vat where it dissolves into green liquor The green liquor is slaked by calcium oxide to produce white liquor and sodium carbonate to furnish sodium hydroxide. The calcium oxide, as a result, is converted to calcium carbonate and white liquor which settles at the bottom of a white liquor clarifier tank. The resulting calcium carbonate, after separation of adhering white liquor, is transported to a lime kiln where the calcium carbonate is converted to calcium oxide. Thus the black liquor recovery cycle is completed.

[0003] Another pulping process employs a solution derived from sulfur burned to produce sulfur dioxide. The sulfur dioxide forms a bisulfate from a base, often magnesium, to form a solution. The solution containing bisulfate is used to depolymerize, disperse and dissolve lignins and create a “sulfite waste liquor.” In the process, soluble lignin sulfonic acid salts are formed from the solution. Recovery of the concentrated “sulfite waste liquor” is performed with a recovery boiler to produce sulfur dioxide and the inorganic base. Accordingly the recovery cycle has been completed

[0004] Thus, it is believed that only elaborate recovery methods have yet been developed.

[0005] This invention relates to a method of recycling a solution containing sodium hydroxide without using a recovery boiler, a lime kiln or production of a smelt.

[0006] Therefore, an object of this invention is to obviate many of the limitations and disadvantages of the prior art to recover black liquor and recover contained components.

[0007] The primary object of this invention is to produce depolymerized lignins significantly free of sodium compounds.

[0008] Still another object of this invention is to produce depolymerized lignins significantly free of sodium compounds for use as a fuel to avoid production of a smelt.

[0009] An additional object of this invention is to provide recycling of a solution containing sodium hydroxide used to produce depolymerized lignins.

[0010] A significant object of this invention is to utilize a multiple-effect evaporator, previously required to concentrate black liquor, to remove water from the solution containing sodium hydroxide.

[0011] A meaningful object of this invention is to subject depolymerized lignins to a flue gas to remove water from the depolymerized lignins.

[0012] One more object of this invention is to separate black liquor from a soda pulping digestion process to produce depolymerized lignins and a solution containing sodium hydroxide

[0013] With the above and other objects in view, this invention relates to the novel features and alternatives and combinations presently described in the brief description of the invention.

PHRASEOLOGY APPLIED IN THE INVENTION

[0014] Lignins in the presence of a solution containing sodium hydroxide, at an elevated temperature, will depolymerize, disperse and dissolve lignins to form a black liquor. The term, depolymerized lignins, is often applied to lignins in a solution containing sodium hydroxide to depolymerize, disperse and dissolve lignins. Lignins are not well structured polymers. When a lignin polymer is fractured, the term “depolymerized lignins” is often utilized even though no definite “lignin monomer” exists. The term “depolymerized lignin” is used interchangeably to designate lignins. Molecular weight of depolymerized lignins contained in a Kraft black liquor is about 2000-3000 according to KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY 1995 FIFTH EDITION VOLUME 15, page 286

[0015] Black liquor containing depolymerized lignins is separated with a difference in molecular weight by a membrane to provide depolymerized lignins and a filtrate containing sodium hydroxide The membrane, for example, can be a diffusion membrane or a nano filtration membrane. Depolymerized lignins and sodium compounds are reacted by an acid to attain a Ph of about 2-4 and cause the depolymerized lignins to become precipitated as decried by KIRK-OTHMER OP. CIT., page 284 and to produce sodium salts in a slurry. Depolymerized lignins are prone to self condensation under acidic conditions. The mixture containing sodium salts and depolymerized lignins is then subjected to a salt splitting operation to remove salts from depolymerized lignins.

[0016] A salt splitting operation often utilizes either a bipolar membrane or ion exchange resins to remove sodium salt from the mixture. When a bipolar membrane is employed, sodium hydroxide and an acid are created from the sodium salt. Salt splitting may be accomplished with ion exchange resins to remove sodium salt from the slurry containing depolymerized lignins. Filtrate, from membrane filtration of black liquor, contains sodium compounds which is then concentrated to remove water and produce a concentrated solution. A multiple-effect evaporator, previously required to concentrate black liquor, is capable of being applied to remove water from the solution containing sodium hydroxide to produce “white liquor” for recycle. The white liquor regularly contains about 40% sodium hydroxide to about 10% sodium hydroxide The resulting concentrated solution is then skimmed of tall oil. Tall oil resembles a soap possessing sodium. The resulting tall oil, after separation from the concentrated solution, is then reacted with an acid to form a salt solution phase and a phase of crude tall oil, as described by KIRK-OTHMER OP. CIT., VOLUME 23, pages 615-622.

[0017] Means to separate the depolymerized lignins will depend on depolymerized lignins molecular properties including molecular size, density and diffusion rate. Membranes can be selected from the group of membranes including diffusion membranes and nano filtration membranes.

[0018] For supplementary particulars on lignins, for example, refer to Pulp And Paper Technology pages 33-36.

[0019] Membranes, providing a means for separation, may employ a diffusion membrane, an ultra filtration membrane or a nano filtration membrane such as described, for example, in bulletins supplied by membrane manufacturers Osmonics and Desal membrane products, Inc. respectively.

[0020] Salt splitting, for example, is described in bulletins provided by several companies including Aqualytics and Dow Chemical Company.

BRIEF DESCRIPTION OF THE INVENTION

[0021] The present invention, in its broadest aspect, is a method to separate depolymerized lignins from black liquor and create a solution containing sodium hydroxide. This invention defines a method to produce depolymerized lignins, substantially free of sodium compounds, and a solution made suitable for reuse to create additional black liquor. Black liquor, substantially separated from the depolymerized lignins, will provide depolymerized lignins for extraction. The depolymerized lignins are then extracted with water to substantially extract water soluble sodium compounds from the depolymerized lignins and will provide an extractate containing water soluble sodium compounds. The extracted depolymerized lignins normally contains sodium compounds. Addition of an acid to the depolymerized lignins will produce water soluble sodium salts within a slurry and a gas containing hydrogen sulfide. Hydrogen sulfide is combined with sodium hydroxide to form sodium sulfide. The slurry of depolymerized lignins containing salt is then converted to a solution of sodium hydroxide and an acid by means of a salt splitting technique. Sodium hydroxide and acid are recovered from the salt and reused in the method. Removal of the salts will provide depolymerized lignins substantially free of sodium compounds. The extractate is combined with the previously separated solution containing sodium hydroxide The combined solution is then concentrated to produce additional black liquor.

[0022] Key features of this invention are:

[0023] Separating depolymerized lignins from sodium compounds contained in black liquor.

[0024] The solution containing sodium hydroxide is subject to recycle for employment to depolymerize, disperse and dissolve additional lignins to form black liquor.

[0025] Production of depolymerized lignins, substantially free of sodium, is suitable to fire boilers to avoid explosions due to the presence of sodium.

[0026] Recycle of substantially all of the sodium compounds as sodium hydroxide and sodium sulfide.

[0027] Recovery of black liquor without utilization of a recovery boiler, a lime kiln or production of a smelt

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] The preferred embodiment of this invention reveals a method to produce depolymerized lignins substantially free from sodium compounds. Black liquor is commonly derived from digestion of wood and is separated from resulting pulp. Black liquor is separated to from depolymerized lignins and a liquid containing sodium hydroxide. The separated depolymerized lignins are then extracted by water to produce an extractate and a raffinate of depolymerized lignins. Sodium compounds contained in the raffinate of depolymerized lignins will be formed into water soluble salts by addition of an acid to form a slurry. Separation of the water soluble salts from the depolymerized lignins slurry will provide depolymerized lignins substantially free of sodium. The solution, previously separated, is combined with the extractate and will be concentrated to produce additional black liquor. Accordingly depolymerized lignins, substantially free from sodium compounds, are produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The features that are considered characteristic of this invention are set forth in the appended claims. This invention, however, both as to its origination and method of operations as well as additional advantages will best be understood from the following description when read in conjunction with the accompanying drawings in which:

[0030]FIG. 1 is a flow sheet denoting the invention as set forth in the appended claims.

[0031]FIG. 2 is a flow sheet denoting an alternate method of the invention.

[0032]FIG. 3 is a flow sheet denoting a method for bipolar membrane salt splitting.

[0033]FIG. 4 is a flow sheet denoting an ion exchange method for salt splitting.

[0034]FIG. 5 is a flow sheet denoting a method to separate tall oil from a concentrated solution.

[0035]FIG. 6 is a flow sheet denoting a method to separate tall oil from a concentrated solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0036] In the preferred embodiment of the present invention a mixture containing depolymerized lignins and sodium hydroxide as black liquor is furnished.

[0037] The flow diagram of FIG. 1 illustrates the general preferred embodiment of the present invention. In the diagram, rectangles represent stages or functions of the present invention and not necessarily separate components. Arrows indicate direction of flow of material in the method.

[0038] Referring to FIG. 1, black liquor containing depolymerized lignins and sodium hydroxide 10, is conveyed to membrane filtration stage 12, where filtrate 14 is formed and removed from black liquor and concentrated depolymerized lignins 16 are formed from membrane filtration stage 12 and forwarded to extraction stage 18. Water 20, is employed for extracting to provide extractate 22 containing sodium hydroxide and raffinate 24 of depolymerized lignins. Raffinate 24 of depolymerized lignins, containing sodium compounds, is transferred to salt formation stage 26, and reacted with acid 28, to form slurry 30 containing depolymerized lignins and sodium salts. Slurry 30, is then transferred to salt splitter stage 32, to substantially remove salts and create depolymerized lignins 34, substantially devoid of sodium compounds. Depolymerized lignins 34, containing remaining sodium salts, may be scrubbed by ion exchange methods to remove remaining sodium salts from depolymerized lignins. Depolymerized lignins 34, substantially devoid of sodium compounds are available for use as a fuel. Extractate 22 is combined with black liquor 10. Extraction stage 18, is customarily operated in a counter flow manner. Acid 28, can be selected from the group of inorganic acids including sulfuric acid and hydrochloric acid including an individual or a combination of these acids thereof Salt formation stage 26, regularly releases hydrogen sulfide as a gas,(not shown in FIG. 1 ), and is reacted with sodium hydroxide to form sodium sulfide. Filtrate 14, is made available to depolymerize, disperse and dissolve additional lignins to form black liquor. Acid 28, when added to raffinate 24, must attain a Ph of about 2-4 within slurry 30 to cause the depolymerized lignins to become precipitated and accordingly form a slurry. Referring to FIG. 2, black liquor containing depolymerized lignins and sodium hydroxide 10, is conveyed to membrane filtration stage 12, where filtrate 14A is formed, and concentrated depolymerized lignins 16 are formed and forwarded to extraction stage 18. Water 20, is employed for extracting to provide extractate 22 containing sodium hydroxide and raffinate 24 of depolymerized lignins. Raffinate 24 of depolymerized lignins, containing sodium compounds, is transferred to salt formation stage 26, and reacted with acid 28, to form slurry 30 containing depolymerized lignins and sodium salts. Slurry 30, is then transferred to salt splitter stage 32, to substantially remove salts and create depolymerized lignins 34, substantially devoid of sodium compounds. Depolymerized lignins 34, containing remaining sodium salts, may be scrubbed by ion exchange methods to remove remaining sodium salts from depolymerized lignins. Depolymerized lignins 34, substantially devoid of sodium compounds are available for use as a fuel. Filtrate 14A containing sodium hydroxide from filtration stage 12, combined with extractate 22, are made available to depolymerize, disperse and dissolve additional lignins to form black liquor.

[0039] Referring to FIG. 3, a flow sheet, is portrayed, denoting a method for bipolar membrane salt splitting. Slurry 30, containing depolymerized lignins and sodium salts, from FIG. 1, is passed to bipolar membrane stage 32A, where the sodium salts are converted and removed as acid 38, and sodium hydroxide 36 is removed from the slurry 30 to create depolymerized lignins substantially free of sodium salts 34A. Formed slurry 34A of depolymerized lignins, containing remaining sodium salts, is scrubbed by ion exchange resins to scrub sodium salts from the slurry and produce depolymerized lignins presumably free of sodium salts.

[0040] Referring to FIG. 4, a flow sheet, is portrayed, denoting an ion exchange method for salt splitting. Slurry 30, containing depolymerized lignins and sodium salts, from FIG. 1, is subjected to ion exchange resins stage 32B, where ions of the salts are transferred to the resins to remove sodium salts and produce depolymerized lignins presumably free of sodium salts 34B.

[0041] Referring to FIG. 5, a flow sheet, is portrayed, denoting a method to produce a concentrated sodium hydroxide solution. Filtrate 14 and extractate 22, from FIG. 2, containing sodium hydroxide, are concentrated and skimmed of tall oil from a solution of concentrated sodium hydroxide. Filtrate 14 containing sodium hydroxide and extractate 22, also containing sodium hydroxide, are conveyed to multiple-effect evaporator stage 40, to remove water 42, and produce concentrated solution 44, containing sodium hydroxide, which is then transferred to tall oil skimming stage 46, to remove tall oil 48, from the concentrated solution 44 containing sodium hydroxide. Produced concentrated solution 50, containing sodium hydroxide, substantially depleted from tall oil, is customarily employed to depolymerized and dissolve additional lignins to create black liquor. The heretofore concentrated sodium hydroxide solution, containing about 40% sodium hydroxide to about 10% sodium hydroxide, may have a further increase of concentration by addition of sodium hydroxide.

[0042] Referring to FIG. 6, a flow sheet, is portrayed, denoting a method to produce a concentrated sodium hydroxide solution. Filtrate 14, from FIG. 1, containing sodium hydroxide, is concentrated and skimmed of tall oil from a solution of concentrated sodium hydroxide. Filtrate 14 containing sodium hydroxide, is conveyed to multiple-effect evaporator stage 40, to remove water 42, and produce concentrated solution 44, containing sodium hydroxide, which is then transferred to tall oil skimming stage 46, to remove tall oil 48, from concentrated solution 44 containing sodium hydroxide. The produced concentrated solution 50, containing sodium hydroxide, substantially depleted from tall oil, is customarily employed to depolymerized and dissolve additional lignins to create black liquor. 

What is claimed is:
 1. A method to produce depolymerized lignins substantially free from sodium compounds, which comprises: providing black liquor containing depolymerized lignins and sodium hydroxide and, providing membrane means for separation of depolymerized lignins from said black liquor, and separating said black liquor from said depolymerized lignins and producing a filtrate containing sodium hydroxide, and extracting with water the heretofore separated depolymerized lignins containing sodium compounds to provide an extractate containing sodium compounds and a raffinate containing depolymerized lignins, and combining said filtrate containing sodium hydroxide with said extractate to form a solution containing sodium hydroxide, and combining an acid with said raffinate of heretofore extracted depolymerized lignins to form a slurry of sodium salts and the depolymerized lignins, and subjecting said slurry to salt spiting to substantially remove salts and produce depolymerized lignins substantially free of sodium whereby depolymerized lignins substantially free of sodium compounds is produced and a solution of sodium hydroxide is produced suitable for recycle to produce black liquor.
 2. The method as described in claim 1 wherein said membrane means for separation of said black liquor is selected from the group of membranes including diffusion membranes and nano filtration membranes including an individual or a combination thereof.
 3. The method as described in claim 1 wherein said slurry containing sodium salts is dependent on salt splitting bipolar membranes to create sodium hydroxide and create an acid from the slurry containing sodium salts and produce depolymerized lignins substantially free of sodium.
 4. The method as described in claim 3 wherein the depolymerized lignins substantially free of sodium is subjected to ion exchange resins to scrub remaining sodium salts from depolymerized lignins to create depolymerized lignins presumably free of sodium salts.
 5. The method as described in claim 1 wherein said slurry, containing sodium salts, is subjected to salt splitting ion exchange resins to remove sodium salts and produce depolymerized lignins presumably free of sodium salts.
 6. The method as described in claim 1 wherein said acid is selected from the group of inorganic acids including sulfuric acid and hydrochloric acid including an individual or a combination thereof.
 7. The method as described in claim 1 wherein the extraction with water is accomplished by weak black liquor obtained from pulp washing.
 8. The method as described in claim 1 wherein said slurry is of Ph about 2 to about
 4. 9. The method as described in claim 1 wherein said solution containing sodium hydroxide is combined with sodium hydroxide to increase concentration of sodium hydroxide within the solution.
 10. The method as described in claim 1 wherein said solution containing sodium hydroxide contains sodium sulfide.
 11. The method as described in claim 1 wherein heretofore combined sodium hydroxide solution is reacted with hydrogen sulfide to form sodium sulfide.
 12. The method as described in claim 1 wherein said black liquor is supplied from kraft pulping digestion.
 13. The method as described in claim 1 wherein said black liquor is supplied from soda pulping digestion.
 14. The method as described in claim 1 wherein said depolymerized lignins substantially free of sodium are subjected to flue gas to remove water from the depolymerized lignins.
 15. The method as described in claim 14 wherein the depolymerized lignins are utilized as a fuel.
 16. The method as described in claim 1 wherein said solution containing sodium hydroxide is concentrated by removal of water by a multiple-effect evaporator to produce a concentrated solution containing sodium hydroxide.
 17. The method as described in claim 16 wherein the concentrated sodium hydroxide solution contains about 40% sodium hydroxide to about 10% sodium hydroxide.
 18. The method as described in claim 16 wherein the heretofore concentrated solution is substantially separated from tall oil contained in the concentrated sodium hydroxide solution. 19 The method as described in claim 18 wherein the heretofore separated tall oil is reacted with an acid to form a phase of sodium salts and a phase of crude tall oil.
 20. The method as described in claim 1 wherein the extraction with water is accomplished by counter flow of water. 